Materials having a positive temperature coefficient (PTC) experience an increase in electrical resistance when their temperature is raised. Materials which have useful engineering applications usually show a relatively rapid increase with temperature, i.e. a higher coefficient. The higher the coefficient, the greater an increase in electrical resistance for a given temperature increase.
PTC ceramics are known in the art. Most ceramics have a negative coefficient, whereas most metals have positive values. While metals do become slightly more resistant at higher temperatures, the PTC ceramics (often barium titanate and lead titanate composites) have a highly nonlinear thermal response, so that it becomes extremely resistive above a composition-dependent threshold temperature. This behavior causes the material to act as its own thermostat, since the material conducts current below a certain temperature, and does essentially not conduct current above a certain temperature.